Exemplary embodiments of the invention relate to a dual-band phased array antenna with built-in grating lobe (GL) mitigation.
In the field of phased array antennas it is well-known that the radiating elements (REs) must have a distance of less than half of the shortest wavelength radiated by the antenna to enable a scanning area of the antenna with a broad beam width. Associated with each radiating element is a phase shifting device or a time delaying device to enable the electronic scanning by the phased array antenna. In modern phased array antennas there are additional power amplifiers for transmission and low noise amplifiers for receiving, as well as RF switches and electronic circuits for control integrated into transmit receive modules (TRMs) behind each radiating element. These antennas are called active electronically scanned arrays (AESA) and consist of a large number of TRMs. It is also well-known that the beam width of an antenna is inversely proportional to the array diameter measured in wavelength. In order to achieve small antenna beams a large number of TRMs is required, which may be expensive.
Performance of a radar with search tasks is mainly characterized by its power-aperture product, where the aperture is built-up of the sum of the radiating element areas. As well-known from the phased array theory, the distance of the radiating elements has to be on the order of half a wavelength or smaller to guarantee a wide grating lobe-free electrical scan angle Θ. The relation between the attainable grating lobe-free scan angle Θ and the corresponding maximal distance d between the radiating elements is as follows:
  d  <      λ    ·                  1                  1          +                      sin            ⁢                          Θ              2                                          .      In the following this relation is referred to as the “λ/2 condition”.
This means that achieving a grating lobe-free scan over the full hemisphere (Θ=180° requires the maximum distance d between the radiating elements to be smaller than λ/2 as mentioned before. If the required grating lobe-free scan angle is smaller, e.g. 90°, the resulting distance d between the radiating elements can be larger (d<0.59*λ).
Antennas with high gain require a relatively high number of radiating elements, which may become expensive taking into account that for each radiating element an associated TRM is needed.
Increasing the size of the radiating elements will result in larger antenna aperture, smaller antenna beams, higher antenna directivity, and better angular resolution but with the drawback of grating lobes, especially at large scanning angles. Lowering the operation frequency would reduce or avoid the grating lobe problems, but antenna beam width would increase, directivity and angular resolution decrease, which is not in favor of exact angular position estimation tasks.
To avoid two separate electronically steered antennas—one for the lower band (e.g. S-Band) and one for the upper band (e.g. X-band)—prior art antennas, as disclosed in the U.S. Pat. No. 7,034,753, use a special partitioning of the array in upper frequency areas and lower frequency areas, whereas in each area an antenna grid is used that fulfills the half wavelength condition. As only the corresponding area is used for each radio frequency no grating lobes are expected in the whole angular scanning area. The disadvantage of this solution is that only a part of the aperture can be used for each operating frequency, with well-known degradations of the radar performance with respect to the detection range.
Suppression or mitigation of grating lobes is also known from prior art. One known solution is the use of the patterns of the radiators to suppress the grating lobe. For arrays that are only steered to boresight of the array, the patterns of the radiators can be designed in this way so that the nulls will coincide with the grating lobe of the array. As a result the grating lobe are significantly reduced. The grating lobe will, however, appear if the array is electronically steered, as the grating lobe will move with the main lobe (ML) whereas the nulls of the radiator will stay, so that the grating lobe will be visible and may become as large as the main beam. To avoid the strong increase of grating lobe during electronically steering of the array, the radiator can be designed to have some overlapping area, so that the pattern of the radiator will become small, that the grating lobe will be outside this pattern as described, for example, in US Patent Document 2014/0375525 A1. A disadvantage of this method is the strongly reduced scanning area for the main beam, as the pattern of the radiator may become very small.
Another method to mitigate the grating lobe of arrays that infringes the half wavelength condition is the use of irregular grids for the arrangement of radiators on the array. In this case the grating lobe will smear over a broad region and therefore the grating lobe will be well below the main beam over a wide scanning area. U.S. Pat. No. 3,811,129 describes such a method for grating lobe mitigation. The disadvantage is that it leads to a difficult manufacturing of irregular arrangements of the radiators, which makes the method very expensive.
A further method to mitigate the system wide impact on radar systems is the special design of the transmit pattern of separate transmit antennas, as disclosed in U.S. Pat. No. 3,270,336. In this case a second antenna is introduced.
In KRIVOSHEEV, Yury V.; SHISHLOV, Alexandr V. “GL suppression in phased arrays composed of identical or similar subarrays”. In: Proceedings of Symposium on Phased Array Systems and Technology. Waltham-Boston. 2010. S. 724-730, where subarrays are displaced, or slightly rotated in a plane arrangement against each other, in order to displace the grating lobe of the subarrays, so that a zero in the grating lobe of the whole array is placed. With these methods grating lobe reduction up to approximately 5 dB is reported. The disadvantage of the method is that the number of subarrays that can be arranged is practically very limited.
Jamnejad, V.; Huang, J.; Levitt, B.; et. al., “Array antennas for JPL/NASA Deep Space Network,” in Aerospace Conference Proceedings, 2002. IEEE, vol. 2, no., pp. 2-911-2-921 vol. 2, 2002 doi: 10.1109/AERO.2002.1035672 explains that for phased array antennas grating lobes can be prevented if the radiating elements are spaced approximately half the wavelength apart. Further, a multi-frequency operation capability of phased array antennas can be achieved by stacking or interleaving array elements at two or more frequencies. In another example, this document describes an arrangement of subarrays on a semi-spherical surface with different boresight normal vectors of each subarray in order to achieve a hemispherical coverage of the antenna beam. Beam scanning is provided by a combination of switching the appropriate subarrays on or off and by providing beam steering of each individual subarray.
European Patent Document 2 613 169 A1 discloses a further method for grating lobe mitigation. This method digitally distinguishes main lobe from grating lobe and side lobe detections by applying receive weights to return radar data for each radar receive element to steer each subarray of an array radar antenna to a direction other than the subarray transmit angle.